The measurements referred to above, i.e., measurements of heat flow and fluid flow magnitude and direction in the formation surrounding a borehole provide information that has a direct bearing on the location and assessment of geothermal resources as well as oil resources, both having a particular thermal or heat flow signature. A need exists for an instrument that will produce such measurements in a relatively rapid, efficient and reliable manner, while providing very high accuracy.
Thermal probes used in the measurement of the ground water flow rate are discussed in Reiter et al, "Measurement of Groundwater Flow using an In-Situ Thermal Probe", New Mexico Water Resources Research Report, WRR1Report 027, Project No. A-042-NMEX, July 1973, and in McLin et al, "Measurement of the Horizontal Component of Ground Water Flow Using a Vertically Positioned In-Situ Thermal Probe", New Mexico Water Resources Research Institute, WRR1Report 055, Project No. A-044-NMEX, May 1975. The thermal probes employed generally comprise an elongate slender metal rod having a heat source along the entire length thereof and a temperature sensor at the midpoint thereof. When a constant quantity of heat is applied to the probe, the rise in temperature caused by the probe is directly related to the rate of water flow past the probe.
Other prior art devices of possible interest include those disclosed in U.S. Pat. Nos. 2,675,702 (Buck); 1,796,547 (Trout); 2,524,150 (Vincent); 2,580,182 (Morgan); 3,808,889 (Rawson); 3,745,822 (Pierce); and 4,109,717 (Cooke). The Buck, Trout, Vincent and Morgan patents are concerned with measuring fluid flow in a producing or test well which, as will appear, is physically a totally different measurement than that provided by the invention. The Pierce patent is concerned with measuring circumferential direction to a heat source but not one in a permeable convection formation. The Cooke patent is concerned with measuring the orientation of fractures around a borehole in response to hydraulic fracturing. The measurement provided, and the purpose thereof, are totally different from that of the present invention. The Rawson patent is relevant to the present invention in that heat flow measurements are made at a local position downhole. However, the Rawson device is adapted for use with conductive or "dry" formations rather than convecting or "wet" permeable formations and cannot be used with the latter because there is little natural thermal gradient to be measured and any fluid in the borehole will short out the brushes of the Rawson device.